An oil injection apparatus used for a typical industrial machine, specifically at a location at which mechanical friction occurs, has become widespread and is now being used in many industrial fields. Although the use of the oil injection apparatus is increasing, many problems associated with the use of the oil injection apparatus occur in industrial sites for reasons described below.
An oil injection apparatus is a device for injecting lubricating oil, i.e. grease, which is made up of a thickener, a base oil, and an additive that are combined physically but not chemically. The thickener is analogous to a sponge and oil is analogous to water. That is, the thickener holds oil as a sponge holds water. When a soaked sponge is squeezed, the sponge releases water. Likewise, when predetermined pressure or stress is applied to grease, oil separation occurs. Measurement of the oil separation is referred to as bleed rate. The higher the bleed rate, the more easily oil separates from the thickener.
There are two types of oil injection apparatuses according to a method of applying pressure to grease to discharge oil. One is a gas discharge method that usually uses nitrogen or hydrogen gas, in which when nitrogen or hydrogen gas is generated by a gas generation device, the generated gas constantly applies pressure to a piston and grease is discharged by the pressure. When the gas continuously presses the piston for a long time, ranging from one month to one year, most of oil is separated from grease and is gradually discharged out of a container. In the end, only a thickener is left and hardened in the container like mud. That is, oil separation progresses to a point at which no oil comes out of the container regardless of the pressure of gas. In this case, since the grease may be misperceived to remain in the container, grease shortage is not properly detected. This is likely to lead to bearing damage.
In order to solve this problem of the gas discharge method, a mechanical discharge method has been developed. There are two mechanical discharge methods widely used in various industrial fields. One is a screw down system and the other is a positive displacement pump system. The screw down system is advantageous over the positive displacement pump system in terms of the bleed rate, but has a limitation in that it has a low grease discharge pressure.
A conventional art of the screw down system is disclosed in U.S. Pat. No. 7,228,941 B2. According to the convention art, a wedge-shaped slide surface is attached to a lower portion of a gear wheel. When the gear wheel rotates, a ram moves up and down along an opposing wedge-shaped slide surface that is connected to the ram and positioned under the wedge-shaped slide surface. As the ram moves up and down, grease is compressed and discharged outside. This oil injection apparatus has a complicated structure and is high in manufacturing cost and energy consumption. Further, since it is necessary to lift the wedge slide with the force of a spring, when flowability of grease is poor due to low temperature or high viscosity of the grease, resistance increases, leading to fluctuation in the amount of grease discharged.
The positive displacement pump system has been developed to solve the problem of the screw down system. The positive displacement pump system takes in and compresses grease by connecting a pump to a small motor. The discharge pressure is increased in this way. This method has an advantage of obtaining a high discharge pressure. However, since a spring continuously presses down grease such that grease can be efficiently introduced into a pump, separation of oil occurs at a certain point as with the gas discharge method. When the oil separation occurs, flowability of grease is deteriorated, and thus the grease cannot be efficiently introduced into the pump. For this reason, an injection hole is likely to clog and grease discharge is stopped.